Struct IgnoredAny

pub struct IgnoredAny;

An efficient way of discarding data from a deserializer.

Think of this like serde_json::Value in that it can be deserialized from any type, except that it does not store any information about the data that gets deserialized.

use serde::de::{
    self, Deserialize, DeserializeSeed, Deserializer, IgnoredAny, SeqAccess, Visitor,
};
use std::fmt;
use std::marker::PhantomData;

/// A seed that can be used to deserialize only the `n`th element of a sequence
/// while efficiently discarding elements of any type before or after index `n`.
///
/// For example to deserialize only the element at index 3:
///
/// ```
/// NthElement::new(3).deserialize(deserializer)
/// ```
pub struct NthElement<T> {
    n: usize,
    marker: PhantomData<T>,
}

impl<T> NthElement<T> {
    pub fn new(n: usize) -> Self {
        NthElement {
            n: n,
            marker: PhantomData,
        }
    }
}

impl<'de, T> Visitor<'de> for NthElement<T>
where
    T: Deserialize<'de>,
{
    type Value = T;

    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        write!(
            formatter,
            "a sequence in which we care about element {}",
            self.n
        )
    }

    fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
    where
        A: SeqAccess<'de>,
    {
        // Skip over the first `n` elements.
        for i in 0..self.n {
            // It is an error if the sequence ends before we get to element `n`.
            if seq.next_element::<IgnoredAny>()?.is_none() {
                return Err(de::Error::invalid_length(i, &self));
            }
        }

        // Deserialize the one we care about.
        let nth = match seq.next_element()? {
            Some(nth) => nth,
            None => {
                return Err(de::Error::invalid_length(self.n, &self));
            }
        };

        // Skip over any remaining elements in the sequence after `n`.
        while let Some(IgnoredAny) = seq.next_element()? {
            // ignore
        }

        Ok(nth)
    }
}

impl<'de, T> DeserializeSeed<'de> for NthElement<T>
where
    T: Deserialize<'de>,
{
    type Value = T;

    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
    where
        D: Deserializer<'de>,
    {
        deserializer.deserialize_seq(self)
    }
}

// Deserialize only the sequence element at index 3 from this deserializer.
// The element at index 3 is required to be a string. Elements before and
// after index 3 are allowed to be of any type.
let s: String = NthElement::new(3).deserialize(deserializer)?;

Trait Implementations

impl Clone for IgnoredAny

fn clone(&self) -> IgnoredAny

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for IgnoredAny

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for IgnoredAny

fn default() -> IgnoredAny

Returns the “default value” for a type.

impl<'de> Deserialize<'de> for IgnoredAny

fn deserialize<D>(deserializer: D) -> Result<IgnoredAny, D::Error>

where D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl PartialEq for IgnoredAny

fn eq(&self, other: &IgnoredAny) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'de> Visitor<'de> for IgnoredAny

type Value = IgnoredAny

The value produced by this visitor.

fn expecting(&self, formatter: &mut Formatter<'_>) -> Result

Format a message stating what data this Visitor expects to receive.

fn visit_bool<E>(self, x: bool) -> Result<Self::Value, E>

The input contains a boolean.

fn visit_i64<E>(self, x: i64) -> Result<Self::Value, E>

The input contains an i64.

fn visit_i128<E>(self, x: i128) -> Result<Self::Value, E>

The input contains a i128.

fn visit_u64<E>(self, x: u64) -> Result<Self::Value, E>

The input contains a u64.

fn visit_u128<E>(self, x: u128) -> Result<Self::Value, E>

The input contains a u128.

fn visit_f64<E>(self, x: f64) -> Result<Self::Value, E>

The input contains an f64.

fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>

where E: Error,

The input contains a string. The lifetime of the string is ephemeral and it may be destroyed after this method returns.

fn visit_none<E>(self) -> Result<Self::Value, E>

The input contains an optional that is absent.

fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>

where D: Deserializer<'de>,

The input contains an optional that is present.

fn visit_newtype_struct<D>( self, deserializer: D, ) -> Result<Self::Value, D::Error>

where D: Deserializer<'de>,

The input contains a newtype struct.

fn visit_unit<E>(self) -> Result<Self::Value, E>

The input contains a unit ().

fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>

where A: SeqAccess<'de>,

The input contains a sequence of elements.

fn visit_map<A>(self, map: A) -> Result<Self::Value, A::Error>

where A: MapAccess<'de>,

The input contains a key-value map.

fn visit_bytes<E>(self, bytes: &[u8]) -> Result<Self::Value, E>

where E: Error,

The input contains a byte array. The lifetime of the byte array is ephemeral and it may be destroyed after this method returns.

fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>

where A: EnumAccess<'de>,

The input contains an enum.

fn visit_i8<E>(self, v: i8) -> Result<Self::Value, E>

where E: Error,

The input contains an i8.

fn visit_i16<E>(self, v: i16) -> Result<Self::Value, E>

where E: Error,

The input contains an i16.

fn visit_i32<E>(self, v: i32) -> Result<Self::Value, E>

where E: Error,

The input contains an i32.

fn visit_u8<E>(self, v: u8) -> Result<Self::Value, E>

where E: Error,

The input contains a u8.

fn visit_u16<E>(self, v: u16) -> Result<Self::Value, E>

where E: Error,

The input contains a u16.

fn visit_u32<E>(self, v: u32) -> Result<Self::Value, E>

where E: Error,

The input contains a u32.

fn visit_f32<E>(self, v: f32) -> Result<Self::Value, E>

where E: Error,

The input contains an f32.

fn visit_char<E>(self, v: char) -> Result<Self::Value, E>

where E: Error,

The input contains a char.

fn visit_borrowed_str<E>(self, v: &'de str) -> Result<Self::Value, E>

where E: Error,

The input contains a string that lives at least as long as the Deserializer.

fn visit_string<E>(self, v: String) -> Result<Self::Value, E>

where E: Error,

Available on crate features std or alloc only.

The input contains a string and ownership of the string is being given to the Visitor.

fn visit_borrowed_bytes<E>(self, v: &'de [u8]) -> Result<Self::Value, E>

where E: Error,

The input contains a byte array that lives at least as long as the Deserializer.

fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>

where E: Error,

Available on crate features std or alloc only.

The input contains a byte array and ownership of the byte array is being given to the Visitor.

impl Copy for IgnoredAny

impl StructuralPartialEq for IgnoredAny